The present invention relates to wavelength division multiplexed (WDM) optical communication systems, in particular, such systems having a tunable laser for performance monitoring.
WDM optical communication systems have been deployed to increase the capacity of existing fiber optic networks. These systems typically include a plurality of transmitters, each including a semiconductor laser diode respectively transmitting signals on a designated one of a plurality of channels or wavelengths. The channels are combined by a multiplexer at one end terminal and transmitted on a single fiber to a demultiplexer at another end terminal where they are separated and supplied to respective receivers. Several parameters, discussed below, effect WDM system performance and should therefore be monitored.
I. Amplifier Gain Flatness
Typically, a plurality of erbium doped fiber amplifiers are provided at nodes spaced along the fiber between the multiplexer and demultiplexer in order to regenerate each channel within the WDM signal. However, erbium doped fiber amplifiers often do not uniformly amplify light across each WDM channel, typically within the spectral region of 1525 to 1570 nm. For example, an optical channel at a wavelength of 1540 nm may be amplified as much as 4 dB more than an optical channel at a wavelength of 1555 nm. While such a large variation in gain can be tolerated for a system with only one optical amplifier, it cannot be tolerated for a system with plural optical amplifiers or numerous, narrowly spaced optical channels. In these environments, much of the amplifier pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths can suffer excessive noise accumulation after propagating through several amplifiers.
II. Chromatic Dispersion
Another factor effecting WDM system performance relates to chromatic dispersion. Optical signals transmitted in a fiber optic communication system typically constitute a series of pulses of digital information. Although the pulses are usually at a single nominal wavelength, each pulse is actually composed different spectral components. These spectral components propagate through the transmission fiber at different speeds, which has the effect of broadening the pulse as it propagates through the fiber. This effect, known as xe2x80x9cchromatic dispersionxe2x80x9d, can result in spectral components of one pulse arriving at a receiver at substantially the same time as a succeeding pulse, thereby causing degraded receiver sensitivity. Dispersion compensated fiber, commercially available from Corning, for example, can be used to offset chromatic dispersion, but its effectiveness over a wide range of wavelengths found in high channel count WDM systems can be limited. Accordingly, due to the wavelength dependence on dispersion, certain WDM channels may be adequately dispersion compensated while others are not.
III. Polarization Mode Dispersion (PMD)
Each optical pulse can further include light of different polarizations in addition to different spectral components. These various polarizations can also propagate at different speeds in the transmission fiber, so that adjacent pulses can bleed into one another making it difficult to accurately detect each pulse.
Each of these effects, as well as others, can reduce system performance, often gauged by measuring an optical signal-to-noise ratio (OSNR) for each channel in the WDM system. Conventional monitoring techniques for measuring OSNR, as well as the above-identified parameters, have implemented a tunable filter at various monitoring points along a WDM system. The tunable filter individually selects each channel of the WDM system, and using well-known methodologies and techniques, gain flatness and OSNR can be measured. Tunable filters, however, are expensive, and the cost for deploying large numbers in a WDM system traversing many hundreds of kilometers can be prohibitive.
Alternatively, subcarrier modulation techniques can be employed in which a unique subcarrier is applied to each WDM channel, thereby allowing each channel to be diagnosed individually through electronic filtering. This approach, however, requires that each WDM channel be present. If no channels are present, the system cannot be monitored.
Consistent with the present invention, an optical device is provided having a first a first optical transmitter emitting a first optical signal having a first wavelength, and a second optical transmitter emitting a second optical signal having a second wavelength different than the first wavelength. The second wavelength is variable among a plurality of wavelengths. An optical combining element is further provided which is configured to combine the first and second optical signals onto a common optical communication path. In addition, an optical filtering element is coupled to the optical communication path having an associated transmission spectrum with a plurality of transmission peaks, each of which corresponding to a respective one of said plurality of wavelengths. A receiver circuit is also provided which is coupled to the optical filtering element, said receiver circuit is configured to sense the second optical signal.
In alternative embodiments, the optical filtering elements may be omitted if the second wavelength is varied in a step-wise fashion to avoid any overlap with the first wavelength or if its optical power is sufficiently low, so that it does not interfere significantly with the first wavelength.